1. Field of the Invention
The invention relates to a photographic camera with means for reducing the focussing error resulting from setting the diaphragm.
2. Description of the Prior Art
In a known camera of this type, as described in West German Disclosure Publication No. 22 57 608.5, the stopping element takes the shape of a brake disk of ferromagnetic material resting on a flexible shaft, along the periphery of which the magnet poles of the electromagnet are arranged with a small opening for the shaft. A pinion is rigidly connected with the brake disk and meshes with a spur rack which, in turn, is placed on the driving element formed as an axially moving actuating rod. The electromagnet is controlled by the exposure meter and receives an exciting current impulse when the diaphragm has reached the position corresponding to the prevailing object brightness, that is to say, the light passing through the remaining lens aperture results in an exact exposure of the photographic image. The magnetic field resulting at the magnet poles during the magnetic induction is closed over the brake disk. The latter is attracted by the flexible shaft and stopped mechanically against the pole areas of the electromagnet. The rotating brake disk thus constitutes the armature of the electromagnet and forms part of the magnetic circuit.
Considerable focusing errors occur in such a stopping device, that is to say, deviations of the actual diaphragm position from the desired diaphragm position at the instant when the stop signal was fed to the electromagnet. These focusing errors are mainly due to the large mass of the brake disk which cannot be stopped instantaneously but only with a definite time lag. The mass of the brake disk, however, can only be minimized to a limited extent because the magnetic flux of the electromagnet, being closed over the brake disk, requires a brake disk with adequate thickness. Furthermore, tolerances between the spur rack and the driving pinion for the brake disk contribute to the lag or the focusing error.